Interferon treatment of cells producing murine luekemia viruses (MLV) or mouse mammary tumor virus (MMTV) resulted in a marked decrease in virus yield. In some systems this appeared to be due to an inhibition in virus budding from the plasma membrane of the cell; in others, virus with a marked decrease in infectivity was produced. Several proteins of MLV seem to be altered in virus derived from interferon-treated cells. In addition, in an RNA virus from an entirely different group, vesicular stomatitis virus (VSV), particles of low infectivity are produced after interferon treatment. In this system, there appears to be an inhibition in the incorporation of the viral glycoprotein into the virion. These changes may be related to interferon induced alterations in the cell plasma membrane. In addition, the mechanism of interferon-induced inhibition of virus protein synthesis in cell-free systems is being studied.